1. Field of the Invention:
The present invention relates to a process for producing high tension steel products, such as plates, sheets and strips (herein called sheets), having an excellent low-temperature toughness with a yield point of 40 kg/mm.sup.2 or higher.
The steel products according to the present invention are useful as hot rolled or as heated at a temperature ranging from 300.degree. to 750.degree. C after the hot rolling.
2. Description of Prior Art:
Conventionally, steel sheets such as for pipe lines in cold climate regions whih are required to have high strength and toughness in the "as rolled" condition have been produced by a method called "controlled rolling" (hereinafter abridged as CR), and mainly Nb-containing steels have been used for this purpose.
Generally, the CR method is composed of two steps; the first step is a heating step and the second step is a rolling step (cooling), if largely classified. The following considerations must be made in these steps, respectively.
(1) In the heating step, it is required to dissolve elements such as Nb and V enough for refinement of the structure and precipitation hardening, and it is required to maintain the austenite grains during the heating (heated .gamma. grains) as fine as possible.
(2) In the rolling step, it is necessary to recrystallize the heated .gamma. grains repeatedly by rolling to obtain refined rolled austenite grains (rolled .gamma. grains), and it is necessary to elongate the rolled .gamma. grains and reduce their thickness by rolling in their non-recrystallization zone so as to obtain refinement of the rolled structure.
However, in case of Nb-containing steels as commonly used, Nb(CN) is stable at high temperatures and it is difficult to ressolve Nb(CN) consistently and satisfactorily even by a long heating time if the heating temperature is not higher than 1150.degree. C.
If the heating temperature is raised, it is possible to attain a satisfactory solid solution of Nb(CN). On the other hand, the heated .gamma. grains grow excessively, thus resulting in considerable deterioration of the toughness of the rolled steel.
Therefore, in the CR method, it is necessary to lower the heating temperature to keep the heated .gamma. grains smaller when severe low-temperature toughness is required. On the other hand, when the heating temperature is lowered, the amount of Nb in solid solution increases or decreases depending on a slight change in the heating temperature and time in case of the commonly used Nb-containing steels. Even under the same rolling conditions, the resultant strength fluctuates in a wide range depending on the change in the amount of the solid solution Nb, and high strength, if obtained, is accompanied with deterioration of toughness. Thus, it is difficult to obtain a stable balance between strength and toughness.
The above difficulties can be attributed to the fact that toughness lowers in proportion to the increase of strength, and the increase of strength corresponds to the increase of the amount of Nb(CN) in solid solution and the coarsening of the heated .gamma. grains, so that the steel structure will be made of coarse and mixed grains.
However, in the conventional CR method, proper consideration has not been given to the fact that the heated .gamma. grains coarsen when enough Nb(CN) is dissolved in solid solution during the heating step, and thus toughness is deteriorated.
As described above, it is necessary to prevent the growth of the heated .gamma. grains by means of the precipitation in order to maintain fine heated .gamma. grains and improve toughness.
For this purpose, it is required to lower the heating temperature and keep the precipitates such as Nb(CN) from solid solution during the heating. On the other hand, in order to maintain the strength, it is necessary to dissolve Nb(CN) into solid solution as much as possible during the heating so as to precipitate it during the cooling after the rolling to strengthen the steel. For this purpose, it is desirable to maintain the heating temperature as high as possible.